1. Field of the Invention
The present invention relates to the retorting of shale in a staged turbulent bed. More specifically, the present invention pertains to a preferred baffle arrangement for such a retort.
2. Description of the Prior Art
A staged turbulent bed retorting process is described in U.S. Pat. No. 4,199,432, incorporated herein, filed Mar. 22, 1978 and issued Apr. 22, 1980, for efficiently and economically retorting shale of a broad particle size distribution. In the process raw shale particles and hot spent shale particles are introduced into an upper portion of a vertically elongated retort vessel and pass downwardly therethrough. Heat transfer from the hot spent shale to the raw shale provides the heat for retorting.
The maximum particle size for the raw shale or previously retorted shale particles in such a process is normally maintained at or below 21/2 mesh, Tyler Standard Sieve size. Particle sizes in this range are easily produced by conventional means such as combinations of cage mills, jaw, or gyratory crushers. Raw shale crushing operations may be conducted to meet a maximum particle size specification, but little or no control is effected over the smaller particle sizes.
The temperature of the spent shale introduced to the retort is normally in the range of 1100.degree. F.-1500.degree. F. A correspondingly appropriate operating ratio of heat carrier to shale is then used to achieve the desired temperature in the retort. The raw shale is introduced at ambient temperature or, if desired, preheated to reduce the heat transfer required between the raw shale and the heat carrier. The temperature at the top of the retort is normally maintained within the broad range, 850.degree. F. to 1000.degree. F., and is preferably maintained in the range of 900.degree. F. to 950.degree. F.
The weight ratio of spent shale heat carrier to fresh shale may be varied from approximately 1.5:1 to 8:1 with a preferred weight ratio in the range of 2.0:1 to 3:1. It has been observed that some loss in product yield occurs at the higher weight ratios of spent shale to fresh shale and it is believed that the cause for such loss is due to increased adsorption of the retorted hydrocarbonaceous vapor by the larger quantities of spent shale. Furthermore, attrition of the spent shale, which is a natural consequence of retorting and combustion of the shale, occurs to such an extent that there is a maximum recycle ratio which can be achieved using spent shale alone. The maximum achievable recycle ratio is dependent on the grade of the fresh shale. If it is desired to operate at higher weight ratios of heat carrier to fresh shale, alternative attrition resistant carriers, such as sand, may be substituted as part or all of the heat carrier.
The mass flow rate of fresh shale through the retort is normally maintained between 5,000 kg/hr-m.sup.2 and 30,000 kg/hr-m.sup.2, and preferably between 10,000 kg/hr-m.sup.2 and 20,000 kg/hr-m.sup.2. Thus, in accordance with the broader recycle heat carrier weight ratios stated above, the total solids mass rate will range from approximately 12,500 kg/hr-m.sup.2 to 270,000 kg/hr-m.sup.2, preferably in the range 25,000-176,000 kg/hr-m.sup.2, and more preferably 30,000-78,000 kg/hr-m.sup.2.
A stripping gas is introduced into a lower portion of the retort and passes upwardly through the vessel in countercurrent flow to the downwardly moving solids. The flow rate of the stripping gas is normally maintained to produce a superficial gas velocity at the bottom of the vessel in the range of approximately 30 cm/second to 150 cm/second, with a preferred superficial velocity in the range of 30 cm/second to 90 cm/second. The stripping gas may be comprised of steam, recycle product gas, hydrogen, an inert gas or any combination thereof. It is particularly important, however, that the stripping gas selected be essentially free of molecular oxygen to prevent product combustion within the retort.
The stripping gas will fluidize those particles of the raw shale and heat carrier having a minimum fluidization velocity less than the velocity of the stripping gas. Those particles having a fluidization velocity greater than the gas velocity will pass downwardly through the retort, generally at a faster rate than the fluidized particles.
An essential feature of the staged turbulent bed retorting system lies in limiting the maximum bubble size and the gross vertical backmixing of the downwardly moving shale and heat carrier to produce stable, substantially plug flow conditions through the retort volume. True plug flow, wherein there is little or no vertical backmixing of solids, allows higher conversion levels of kerogen to vaporized hydrocarbonaceous material than can be obtained, for example, in a fluidized bed retort of equivalent volume where there is gross top to bottom mixing. Maintaining substantially plug flow conditions by limiting top to bottom mixing of solids, allows one to operate the retorting process on a continuous basis with a much greater control of the residence time of individual particles. The use of means for limiting substantial vertical backmixing of solids also permits a substantial reduction in size of the retort zone required for a given mass throughput, since the chances for removing partially retorted solids with the retorted solids are reduced. The means for limiting backmixing and limiting the maximum bubble size are generally described as baffles, barriers, dispersers or flow redistributors, and may, for example, include spaced horizontal perforated plates, bars, screens, packing, or other suitable internals.
Gas bubbles tend to coalesce in the staged fluidized bed to form larger bubbles. Oversized bubbles cause surging or pounding in the bed, leading to a significant loss of efficiency in contacting and an upward spouting of large amounts of material at the top of the bed. The means provided for limiting backmixing also limits the coalescence of large bubbles, thereby allowing the size of the disengaging zone to be somewhat reduced.
Although gross vertical backmixing should be avoided, highly localized mixing is desirable in that it increases the degree of contacting between the solids and the solids and gases. Localized mixing necessarily introduces some vertical mixing and thus deviates from strictly plug-flow behavior. The degree of backmixing is dependent on many factors, but is primarily dependent upon the particular internals or baffles disposed within the retort.
Of great importance in the staged turbulent bed process is the interaction between the fluidized solids, the non-fluidized solids, and the means employed for preventing backmixing. The fluidized solids generally proceed down the retort as a moving fluidized columnar body. Without internals, a stable fluidized moving bed cannot be achieved with a solids mixture having the broad particle distribution of unclassified shale. The means to limit backmixing significantly affects the motion of the non-fluidized particles and thereby substantially increases the residence time of said particles. The average velocity of the falling non-fluidized particles, which determines said particles' residence time, is substantially decreased by momentum transfer to the fluidized solids and the retort vessel internals. This increased residence time thereby permits the larger particles to be retorted in a single pass through the vessel. It has been discovered that with some internals, such as horizontally disposed perforated plates having a 49% free area and spaced throughout the vessel at eight-inch spacings, the residence time of the non-fluidized particles will approach the average particle residence time.
A retort having overall plug flow characteristics, and intense local mixing provides the equivalent of a serial plurality of perfectly mixed stages. The term "perfectly mixed stage" as used herein refers to a vertical section of the retort wherein the degree of solids mixing is equivalent to that attained in a perfectly mixed volume having gross top-to-bottom mixing. The number of equivalent perfectly mixed stages actually attained depends upon many inter-related factors, such as vessel cross-sectional area, gas velocity, particle size distribution and the type of internals selected to limit gross top-to-bottom backmixing. It is preferred that the retort provide the equivalent of at least four perfectly mixed stages.
Excellent stripping of the hydrocarbonaceous vapor from the retorted solids is uniquely achieved with the staged turbulent bed retort. With the staged flow characteristics, the "lean" stripping gas first contacts those particles having the least amount of adsorbed hydrocarbonaceous material, thus maximizing the driving force for mass transfer of the hydrocarbonaceous vapor into the fluidization stream.
Due to the hydrocarbon vapors evolved from the shale which mix with the stripping gas, the gas velocity increases along the length of the column. The actual amount of increase will depend upon the grade of shale processed and the mass rate of fresh shale per unit cross-sectional area, but may be minimized, if necessary, by proper initial design of the retort vessel. In this regard, the vessel may have an inverted frustoconical shape or may be constructed in sections of gradually increasing diameter.
The pressure at the top of the retort is preferably maintained no higher than that which is required to accommodate downstream processing. The pressure in the bottom of the retort will naturally vary with the chosen downstream equipment, but will normally be in the range of 2-4 atmospheres.
A product effluent stream comprised of hydrocarbonaceous material admixed with the stripping gas is removed from the upper portion of the retort by conventional means and passes to a conventional separation zone. Since the product effluent stream will normally contain some entrained fines, it is preferred that said fines be separated from the remainder of the stream prior to further processing. This separation may be effected by any suitable or conventional means, such as cyclones, pebble beds and/or electrostatic precipitators.
The retorted shale along with the spent shale serving as heat carrier is removed from the lower portion of the retort by conventional means at the retort temperature. The retorted shale will normally have a residual carbon content of approximately 2 to 4 weight percent and represents a valuable source of energy which may be used to advantage in the process.
The retorted shale and spent shale are fed to a lower portion of a combustor which may be of any conventional design, but it is preferred to use a dilute phase lift combustor. Air is injected into the lower portion of the combustor and the residual carbon on the shale is partially burned. The carbon combustion heats the retorted shale to a temperature in the range of 1100.degree. F. to 1500.degree. F. and the hot shale and flue gas are removed from the upper portion of the combustor. A portion of said hot shale is recycled to provide heat for the retort. Preferably said recycled shale is classified to remove substantially all of the minus 200 mesh shale prior to introduction to the retort to minimize entrained fines carryover in the effluent product vapor.